You didn't mention if the Urinary diet was canned or kibble, if it's kibble, stop feeding it. Canned will help with constipation enormously (and your obese cat). Second, the food is probably overly priced, not so great quality and shouldn't be fed to a cat.

A great food is Wellness, grain free, canned, and you can buy it in the larger cans to help with the cost. It will also be a low residue food as it is a higher quality. If you want the lowest residue food, then raw is the way to go. Their feces become quite small when fed it.

A quality food will also help with all of their immune systems.

I have to run, but will be back for more advice. Hopefully Sugarcatmom will also be on to help you out.